There are certain physical conditions or parameters whose levels must be measured with relatively good precision. For example, if the condition is some type of air pollutant for which a maximum has been established, it is important to accurately determine the level reliably and cheaply so as to allow remedial actions (which may be very expensive) whenever needed and yet not fail to take such actions either whenever required. There are other situations where these parameters are under the control of some system or device such as a HVAC (heating, ventilating and air conditioning) system where accurate measurement is necessary to reduce operating cost and yet assure occupant comfort. In this second example, it is commonly desirable to measure certain of these parameter levels with precision and to provide the measurements in electronic signals as a feedback to allow a control system to control these values with precision. Two of the most important of these parameters are relative humidity and temperature.
In electronically based parameter value measuring systems it is common to use as a sensor element an electronic component such as a resistor or capacitor whose electrical value changes with the parameter value. In particular, there are relative humidity and temperature sensor elements whose capacitance or resistance changes in a predictable manner with the change in the value of the parameter involved. It is thus possible to measure this parameter value by measuring the capacitance or resistance of the sensor element.
Measuring the capacitance or resistance of such a sensor element using only a cheap and simple circuit is not an easy job however. One common approach is to use an RC circuit incorporating the sensor component and a fixed value for the other component and measure the time required to charge the capacitor from a first to a second voltage and from this time calculate the resistance or capacitance. By knowing the relationship between the sensor component value and the parameter value to be measured, one can calculate the parameter value directly from the charging time interval. One can see that the accuracy with which the parameter value is calculated depends on the accuracy with which is measured the first and second voltage levels defining the charging time interval, accurate knowledge of the fixed component value in the RC circuit, and the stability of the timing element which measures the time interval between the two voltage levels. It is necessary if high precision is desired to eliminate all of these effects when measuring these parameter values in the manner explained.
In particular, relative humidity has been measured by using in such an RC circuit a sensor capacitor whose capacitance changes in direct proportion to the ambient relative humidity. However, the various sources of error mentioned have heretofore prevented highly accurate measurements of relative humidity using such a sensor in a conventional RC circuit.
U.S. Pat. No. 4,703,886 (Kirby) is an available reference which uses a sensor element whose capacitance changes as a function of relative humidity. U.S. Pat. No. 4,793,187 (Kordts) describes a circuit which compensates for temperature dependency in a capacitive sensor which measures various types of parameter values.